A variety of image-display/image-projection devices and techniques are available for displaying/projecting graphical or video images—often called video frames—to a viewer. A graphical image, i.e., a graphic, typically changes infrequently or not at all. For example, a flight-instrument graphic of cockpit instruments may overlay a pilot's view. This graphic may be projected onto a viewing area such as the windshield, or may be projected directly into the pilot's eyes such that he/she sees the flight instruments regardless of his/her viewing direction. Typically, there is little change in this graphic other than the movement of the instrument pointers or numbers. Conversely, video frames are a series of images that typically change frequently to show movement of an object or the panning of a scene. For example, a television displays video frames.
A cathode-ray-tube (CRT) display, such as used in a television or computer monitor, is a common image-display/image-projection device that, unfortunately, has several limitations. For example, a CRT is typically bulky and consumes a significant amount of power, thus making it undesirable for many portable or head-mounted applications.
Flat-panel displays, such as liquid-crystal displays (LCDs), organic LEDs, plasma displays, and field-emission displays (FEDs), are typically less bulky and consume significantly less power than a CRT having a comparable viewing area. But flat panel displays often lack sufficient luminance and adequate color purity or resolution for many head-mounted applications.
Referring to FIG. 1, although a scanned-beam display system 71 often overcomes the limitations of the above-described displays, the viewer may lose sight of the displayed image if he/she moves his/her eye 73. The display system 71 includes a scanning source 72, which outputs a scanned beam of light that is coupled to a viewer's eye 73 by a beam combiner 74. In one embodiment, the scanning source 72 includes a scanner (not shown), such as a scanning mirror or acousto-optic scanner, that scans a modulated light beam through a viewer's pupil 75 and onto a viewer's retina 76. In another embodiment, the scanning source 72 may include one or more light emitters (not shown) that are rotated through an angular sweep. Because such displays scan or project an image through the pupil of the viewer's eye, the display's “exit pupil”—defined as an area, often a plane, in front of the viewer's eye 73 where the image is located—is limited to the diameter of the viewer's pupil 75, which typically ranges from about 2 millimeters (mm) in bright light to about 7 mm in dim light. Consequently, the viewer may “lose” the image when he/she moves his/her eye 73. A display system similar to the display system 71 is further described in U.S. Pat. No. 5,467,104, which is incorporated by reference.
Referring to FIG. 2, a scanned-beam display system 82 overcomes the problem of “losing” an image due to eye movement by including a diffraction grating 84 to generate an exit pupil 86, which includes an array of multiple exit-pupil images 88. Specifically, a modulated light beam 92 scans an image 93 onto the diffraction grating 84, where the size of the image is determined by a scanning angle 2θ. The grating 84 diffracts the beam 92 into fractional beams 98a-98c, which respectively generate exit-pupil images 88a-88c as the beam 92 scans the image 93. Each of the images 88a-88c is a replica of, but has a lower intensity than, the image 93. An eyepiece 95 collimates the images 88a-88c to form the exit pupil 86. When the viewer's pupil 75 is aligned with one or more of the images 88a-88c, the aligned image or images 88 converge on an area 100 of the viewer's retina 76 to replicate the image 93. The intensity of the replicated image is proportional to the number of images 88 that converge to form the replicated image on the retinal area 100.
By including multiple exit-pupil images 88, the exit pupil 86 effectively increases the viewer's field of view with respect to the image 93. That is, as long as at least one of the exit-pupil images 88a-88c is within the viewer's field of view, he/she can see the image 93. For example, if the viewer looks down slightly, the exit-pupil image 88b moves out of his/her view, but the image 88a remains in view and the image 88c enters his/her view. Therefore, even though the viewer has moved his/her eye 73, he/she still views the image 93 via the exit-pupil images 88a and 88c. A scanned-beam display system that is similar to the display system 82 is further described in U.S. Pat. No. 5,701,132, which is incorporated by reference.
Unfortunately, the exit-pupil images 88 generated by the scanned-beam display 82 often have non-uniform intensities, which may annoy or distract the viewer. Specifically, the diffraction grating 84 is typically designed for a single wavelength of light, but the image beam 92 typically includes other wavelengths in addition to this single wavelength. These other wavelengths often cause the exit-pupil images 88 to have different intensities. Therefore, one typically limits the intensity of the beam 92 so that the brighter exit-pupil images 88 are not too bright for the viewer. But this may cause some of the dimmer images 88 to be too dim for the viewer to see, thus causing “holes” in the exit pupil 86. Furthermore, even if none of the images 88 are too dim for the viewer to see, the differences in intensity among the images 88 may annoy or distract the viewer as he/she shifts his/her gaze.